1. Field of the Invention
The present invention relates to a monolithic ceramic capacitor and a method for adjusting an equivalent series resistance thereof. In particular, the present invention relates to a monolithic ceramic capacitor including external electrodes which function as a resistance element and a method for adjusting an equivalent series resistance thereof.
2. Description of the Related Art
Heretofore, a monolithic ceramic capacitor has been used to eliminate high-frequency voltage fluctuations of a smoothing circuit, whereas a tantalum capacitor or an aluminum electrolytic capacitor has been used for eliminating low-frequency voltage fluctuations.
As described above, the monolithic ceramic capacitor has been used merely for eliminating high-frequency voltage fluctuations because the equivalent series resistance (ESR) of the monolithic ceramic capacitor is less than those of the tantalum capacitor and the aluminum electrolytic capacitor and, therefore, the components defining the equivalent circuit of the monolithic ceramic capacitor becomes substantially merely a C component and an L component, so that oscillation occurs easily in response to low-frequency voltage fluctuations and noise is generated. Consequently, in order to eliminate low-frequency voltage fluctuations, a capacitor, which does not oscillate in response to low-frequency voltage fluctuations, having a large ESR, that is, the above-described tantalum capacitor or the aluminum electrolytic capacitor must be used.
However, the tantalum capacitor and the aluminum electrolytic capacitor are large as compared to the monolithic ceramic capacitor, and miniaturization of an electronic apparatus provided with such a capacitor is inhibited.
On the other hand, a method, in which a resistance element defined by a discrete component is connected to a monolithic ceramic capacitor in series and, thereby, a circuit is provided with a resistance component so as to suppress oscillation in response to low-frequency voltage fluctuations, is also used. However, in this case, a mounting area for connecting the resistance element defined by a discrete component is required. This also inhibits miniaturization of the electronic apparatus.
In order to solve the above-described problems, it has been proposed to provide a function as a resistance element to external electrodes included in a monolithic ceramic capacitor.
For example, Japanese Unexamined Patent Application Publication No. 4-337616 (Patent Document 1) describes that a metal oxide film is formed on an external electrode surface, and the thickness of the metal oxide film is changed by processing, e.g., barrel polishing, so as to adjust the ESR.
Japanese Unexamined Patent Application Publication No. 11-121276 (Patent Document 2) describes that an intermetallic compound between an external electrode and Sn is formed so as to adjust the ESR.
Japanese Unexamined Patent Application Publication No. 2001-223132 (Patent Document 3) describes that an external electrode is configured to have a three-layer structure composed of a first electrically conductive layer formed from an oxidation-resistant metal, a second electrically conductive layer which is disposed thereon and which is a mixture of an electrically conductive oxide and an insulating oxide, and a third electrically conductive layer which is disposed thereon and which is formed from an oxidation-resistant metal so as to increase the ESR.
In an example described in Patent Document 3, the first electrically conductive layer is formed by baking in a N2 or N2/H2 atmosphere. The second electrically conductive layer primarily includes ruthenium oxide, a ruthenium oxide compound, or graphite, and is formed by baking in air. The third electrically conductive layer has a primary component including at least one type of metal selected from Pd, Ag, Pt, Au, Rh, Ir, and Ru, and is formed by baking in air.
However, individual technologies described in the above-described Patent Documents 1 to 3 have the following problems.
The technology described in Patent Document 1 has a problem in that the adjustment of the ESR is relatively difficult because the ESR is adjusted by the thickness of the metal oxide film.
The technology described in Patent Document 2 has a problem in that it is difficult to attain an adequate ESR because the resistivity of the intermetallic compound is relatively small.
In the technology described in Patent Document 3, the external electrode is a thick film and has a three-layer structure, with each layer being formed by baking. Therefore, the thickness of the entire external electrode is increased, and miniaturization of the component is inhibited. Furthermore, since the oxidation-resistant metal is a noble metal, the material costs of the first and the third electrically conductive layer formed from the oxidation-resistant metal are increased.
In the technology described in Patent Document 3, if ruthenium oxide is used as the material for the second electrically conductive layer, baking in an oxidizing atmosphere, e.g., in air, is indispensable because reduction occurs due to firing in a neutral or reducing atmosphere, e.g., in the N2 atmosphere. However, in order to endure the baking in the oxidizing atmosphere, an expensive noble metal must be used as the material for the internal electrode disposed in the capacitor main body, and it becomes impossible to use Ni or an Ni alloy, which is presently a commonly used material, as the material for the internal electrode.
In the case where a resistant component, e.g., a ruthenium oxide compound which is not reduced, is used as the material for the above-described second electrically conductive layer, a layer for ensuring the continuity to the internal electrode is further required because a highly reliable electrical connection is not ensured between the second electrically conductive layer and the internal electrode. The above-described first electrically conductive layer has a function of ensuring the continuity to the internal electrode as well. Therefore, in the technology described in Patent Document 3, the first to third electrically conductive layers are indispensable, and an increase in the thickness of the external electrode cannot be avoided.